1. Field of the invention
The present invention relates to a process for producing an information recording medium. More particularly, it relates to a process for producing an information recording medium comprising joining a disc-shaped resin film to a disc-shaped resin substrate.
2. Description of prior art
Information recording medium using high energy density beam such as a laser beam have been developed in recent years and are now put into practical use. The information recording medium is called optical disc and can be used as video disc, audio disc and disc memory for large-capacity static image files and large-capacity computers.
The optical image files basically comprises a recording layer provided on a disc-shaped transparant substrate of a plastic material or glass. An undercoat layer or intermediate layer of a high-molecular material can be provided on the surface (on which the recording layer is provided) of the substrate from the viewpoints of improving smoothness of the surface, adhesion between the substrate and the recording layer and the sensitivity of the optical disc.
The recording layer is susceptible to external influences so that it must be protected from the external influences. Thus, there are generally used optical discs having an air sandwich structure where a substrate having a recording layer thereon is joined to another substrate which may have a recording layer via inner and outer spacers in such a state that the recording layer is positioned inwards.
However, the optical disc having an air sandwich structure has disadvantages in that the manufacturing process thereof is complicated and, as a result, the manufacturing cost thereof is high and further in that the thickness of the disc is large and hence it may give difficulty in handling.
In order to solve the above-described problems, there was proposed a method in which a disc-shaped resin film is joined to a surface of the substrate, on said surface side of substrate being provided the recording layer (see, Japanese Patent Provisional Publication No. 51(1976)-75523). In this method, the film is joined to the disc substrate by means of an adhesive in the course of assembly of the optical disc. In order to bond the members to one another by an adhesive, it takes from tens of seconds to several minutes to coat and cure the adhesive for the preparation of one disc, consequently there is difficulty in mass-producing the disc. Further, there is possibility that the adhesive is squeezed out on the outer and inner peripheries of the substrate or on the side on which the recording layer is provided, when the film is joined to the substrate. When the adhesive is squeezed out on the recording layer side of the substrate, the adhesive sticks to the recording layer and as result, the recording layer may deteriorate. Further, there is a problem that the sheet-form film is apt to be peeled off from the substrate by environmental change or with the passage of time, when they are joined to each other by means of an adhesive.
Japanese Patent Provisional Publication No. 61(1986)-104375 proposes a joining method wherein the sheet-form film is joined to the substrate by means of sewing, baking, adhesive tape, pining and nailing in addition to the use of the adhesive.
However, these methods cause the following problems. The joining by pinning or nailing has problems in that the manufacturing process is complicated and there is possibility that the film is broken during handling at a position where the film is fixed, since the film is thin. The joining by sewing or adhesive tape has also disadvantages in that the manufacturing process is complicated, thread or tape is aged with the passage of time and the joint deteriorates or the appearance is spoiled.
The joining by baking, namely, thermal welding in general, is a method of joining the resin film to the substrate by dissolving the film, which can keep good joining of the resin film to the substrate over a long period of time. However, the present inventor has confirmed that in case of thermal welding being made to join the resin film to the substrate in both an inner-side non-recording zone provided around the periphery of the hole and an outer-side non-recording zone provided inside the outer periphery of the substrate, the welding requires large heat so that there is caused heat srinkage on the resin film after the joining by baking. The shrinkage likely causes deformation such as a warp of the substrate.